How to apply wind load on water tank structure?

I am trying to analysis an water tank structure, height of structure is 36 m. when i am trying to apply wind load on structure, load acting up to 18 m only above the 18 m wind load not acting, Why? Can you help me to solve this problem? Up to 21 m structure made up of concrete and after that plate.

The top panel i.e. 18 m to 21m is not having any closed polygon. Without closed polygon wind load generation is not possible. So, please check the member connectivity at 21m level and change in such a way that closed polygon is formed there. then only you will get wind loading there.

The top panel i.e. 18 m to 21m is not having any closed polygon. Without closed polygon wind load generation is not possible. So, please check the member connectivity at 21m level and change in such a way that closed polygon is formed there. then only you will get wind loading there.